An inkjet printer is known as a representative example of a liquid ejection device. The inkjet printer includes: a main printer unit including a mounting unit to which an ink cartridge is removably mounted, a head configured to eject ink, and an ink supplying path configured to supply ink to the head; and the ink cartridge removably mounted to the mounting unit of the main printer unit. The ink cartridge has an ink storage space defined therein, an ink supplying port configured to allow the ink to be supplied from the ink storage space to an outside of the ink cartridge, and an atmosphere communicating port configured to introduce an atmosphere into the ink storage space.
In the inkjet printer, air may enter into the ink supplying path even when the ink cartridge is not mounted in the process of mounting or removing the ink cartridge. For example, JP-A-2005-66906 discloses an inkjet printer (main printer unit) including a sub tank opened to the atmosphere and disposed between the ink cartridge and the ink supplying path so that air does not enter into ink supplying path even when the ink in the ink cartridge is used up. This sub tank has an internal space configured to store the ink, an ink inflow port connected to the ink supplying port of the ink cartridge in a state where the cartridge is mounted, an ink outflow port connected to an upstream end of the ink supplying path, and an atmosphere opening port configured to allow the internal space to communicate with the atmosphere.